Commit-ID:  61e02392d3c7ecac1f91c0a90a8043d67e081846
Author:     Will Deacon <>
AuthorDate: Tue, 13 Feb 2018 13:30:19 +0000
Committer:  Ingo Molnar <>
CommitDate: Tue, 13 Feb 2018 14:55:53 +0100

locking/atomic/bitops: Document and clarify ordering semantics for failed 

A test_and_{}_bit() operation fails if the value of the bit is such that
the modification does not take place. For example, if test_and_set_bit()
returns 1. In these cases, follow the behaviour of cmpxchg and allow the
operation to be unordered. This also applies to test_and_set_bit_lock()
if the lock is found to be be taken already.

Signed-off-by: Will Deacon <>
Acked-by: Peter Zijlstra (Intel) <>
Cc: Linus Torvalds <>
Cc: Paul E. McKenney <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
 Documentation/atomic_bitops.txt   | 7 ++++++-
 include/asm-generic/bitops/lock.h | 3 ++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt
index 5550bfdc..be70b32 100644
--- a/Documentation/atomic_bitops.txt
+++ b/Documentation/atomic_bitops.txt
@@ -58,7 +58,12 @@ Like with atomic_t, the rule of thumb is:
  - RMW operations that have a return value are fully ordered.
-Except for test_and_set_bit_lock() which has ACQUIRE semantics and
+ - RMW operations that are conditional are unordered on FAILURE,
+   otherwise the above rules apply. In the case of test_and_{}_bit() 
+   if the bit in memory is unchanged by the operation then it is deemed to have
+   failed.
+Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
 clear_bit_unlock() which has RELEASE semantics.
 Since a platform only has a single means of achieving atomic operations
diff --git a/include/asm-generic/bitops/lock.h 
index bc39757..67ab280 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -7,7 +7,8 @@
  * @nr: Bit to set
  * @addr: Address to count from
- * This operation is atomic and provides acquire barrier semantics.
+ * This operation is atomic and provides acquire barrier semantics if
+ * the returned value is 0.
  * It can be used to implement bit locks.
 #define test_and_set_bit_lock(nr, addr)        test_and_set_bit(nr, addr)

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